The present invention relates to machinery involving the conduction of electrical current between parts moving relative to each other, more particularly to methods and devices for effecting or facilitating such electrical conduction.
Various kinds of motors, generators and other electrical apparatus require the conduction of electricity between two relatively moving parts. Such mechanical arrangements usually involve the conduction of current between a stationary part (stator) and a rotating part (rotor). A device known as a “brush” or “current collector” is normally used for making sliding contact between stationary and rotating parts so as to conduct electrical current therebetween. A conventional current collection assembly includes a brush and a “holder” (for the brush) as two separate components that are attached to each other. The holder is also attached to either the stationary part or the rotating part of the machinery.
Depending on the particular machinery, a brush can be used to conduct current in either direction (i.e., either from the stationary part to the rotating part, or vice versa), and can be fixed with respect to either the rotating part or the stationary part. Among the desirable qualities of a brush are high current-carrying capacity (e.g., in terms of capability of carrying a high amount of current per unit area of the interface between the brush and the surface contacted thereby), low resistance, low friction, and high wear resistance. Current collection brush technology has grown in interest with the advent and continued development of homopolar machine technology, particularly in the realm of homopolar motors (which operate on direct current) such as those that are currently envisioned for naval ship propulsion.
Conventional brushes include solid (e.g., carbon) brushes and metal fiber (e.g., copper) brushes. The majority of brushes currently used are of the solid carbon variety. Solid carbon brushes provide limited power densities due to their characteristically small number of contact spots. In addition, solid carbon brushes tend to have a short life and to produce conductive wear debris, resulting in frequent brush replacement and frequent machinery cleaning and associated high maintenance costs. Generally speaking, as compared with solid carbon brushes, copper fiber brushes are considered to afford superior performance. However, copper fiber brushes are currently expensive to produce and can support only moderate current densities.
Furthermore, fiber brushes are prone to wear, frequently manifested as a fiber brush “wear-in” contour that matches the curvature of the rotor that is in constant running contact with the fiber brush. The fiber brush wear, sometimes resembling welding-like damage, may result from friction and/or from electrical sparking associated with the fiber brush's sliding contact with the relatively moving machine part. Moreover, the electrical conductivity of fiber brushes is characterized by discontinuities concomitant with the intermittencies of contact by the fibers with the relatively moving machine part. Such contact intermittencies are occasioned by the running nature of the contact in conjunction with the roughness, often microscopic, of the surface of the relatively moving machine part. It has been estimated that a typical brush fiber is in actual physical contact with the relatively moving machine part only about one-third of the time during which the machinery is in operation; hence, each fiber represents a non-contributor to the overall electrical conduction during about two-thirds of the period of machine operation.
Liquid metal additives have been investigated for use in conjunction with fiber brushes in order to alleviate the above-noted conductive intermittency. Although a liquid metal material can succeed in lending constancy or smoothness to the electrical conduction between the brush and the slidingly contacted machine part, the high electrical conductivity of the liquid metal material invites electrical problems in the machinery such as involving short circuiting. Liquid metal brushes are capable of supporting very high current densities, but more research is needed in this area because of problems concerning stability and reactivity. Liquid metals have been tested, with some success, in association with small brushes for high performance current collection in homopolar motors. Generally, liquid metals require very clean operating environments absent oxygen and water. Since liquid metals are highly conductive, unwanted electrical connection of adjacent slip rings can occur (especially at locations where the liquid metal may drip or migrate, such as the bottom of the machine), resulting in short circuits in the machine.
Notable liquid metals that have been tested in current collection context are sodium-potassium (NaK) alloy and various gallium alloys. NaK (an alloy of sodium with potassium, approximately 22% Na and 78% K) has performed well in a range of applications, including both generators and motors. NaK does not react with most materials normally used in electrical machines; however, NaK is a water-reactive, caustic alkali metal, so spillage of this hazardous material is to be avoided. Gallium alloys have similar conductivity attributes, but because of their higher densities and viscosities these metallic materials are best suited for use with low speed propulsion motors. Because gallium alloys slowly react with copper (possibly by forming an alloy with the copper), all copper surfaces must be plated with a suitable non-reactive metal; nevertheless, gallium alloys are non-hazardous and should be further evaluated for current collector applications.